The invention is concerned with wastewater treatment and especially efficient removal of refractory biodegradable compounds including so called microconstituents from wastewater in a membrane bioreactor (MBR) process.
A great deal of research has been undertaken to characterize the ability of conventional activated sludge (CAS) and membrane bioreactor (MBR) technologies to remove microconstituents. Microconstituents are dissolved pollutants that are usually measured on the parts per billion (ppb) or parts per trillion (ppt) level. They include personal care products, pharmaceutical materials and hydrocarbons. Microconstituents are often refractory, long-chain organic compounds that are difficult to degrade biologically given typical solids residence times (SRT) (less than 30 days).
In conventional activated sludge processes using sedimentation or membrane filtration for removal of suspended solids, post-disinfection is used to inactivate or kill pathogenic organisms. In addition to disinfection, post treatment including high pressure filtration (e.g. reverse osmosis) is sometimes employed to remove microconstituents.
Submerged MBR (sMBR) technology has a unique advantage over CAS systems using sedimentation for the separation of solids for biologically treated wastewater in that activated sludge concentrations can be more than three times higher allowing for a longer SRT given the same volume. Research suggests that running at longer SRT can lead to better removal of some refractory compounds and specifically some microconstituents. However, results are mixed and studies have not shown a sufficient correlation between treatment efficiency and SRT; therefore, the efficacy of MBR as compared to CAS Systems remains unquantifiable.
Prior art, whether CAS or MBR, often involves the use of high-pressure filtration such as reverse osmosis (RO) followed by post-oxidation (or post disinfection) of permeate using one or more oxidative compounds. The list includes ozone, chlorine and ultraviolet (UV) radiation. High-pressure permeate filtration, in some cases followed by oxidative post-disinfection, has been successful in destroying some microconstituents but is expensive and in many cases impractical. Moreover, the use of chlorine can lead to the formation of undesirable disinfection byproducts, some of them known carcinogens.